Steerable Percussion Air Drilling System

Huy Bui (hbui@smith-intl.com; 281-233-5428)John Meyers (jmeyers@smith-intl.com; 281-233-5563)

Shantanu Swadi (sswadi@smith-intl.com; 281-233-5220)Smith International, Inc.

P.O. Box 60068Houston, TX 77205-0068

Introduction

Conventional percussion air drilling method, on one hand, yields very high penetration ratesespecially in hard formations, but limits itself to straight hole drilling because it depends on therotation of the entire drill string. On the other hand, downhole motors enable the directionalcontrol of the well bore but cannot penetrate hard rock nearly as fast because they rely on theheavy weight of the drill collars to apply a steady crushing force onto the formation as opposed tothe highly concentrated percussive energy of downhole hammers [1][3][4]. The SteerablePercussion Air Drilling System is an attempt to combine the high penetration rate of percussionair drilling with the directional control techniques of conventional drilling methods. Such asystem would allow directional drilling in hard rock at high penetration rates. The key feature ofSPADS is a rotational drive mechanism that induces an indexing rotational motion onto the bitindependently from the drill string rotation.

Objectives

The developmental effort behind the Steerable Percussion Air Drilling System was defined by thefollowing broad objectives:

To develop a drilling machine capable of disintegrating hard rock by percussive methodand independent of the drill string rotation, i.e., a downhole hammer/motor combinedtogether;

To develop necessary Bottom Hole Assembly components, namely bent subs, stabilizersand a modeling software to enable the prediction and control of the direction of the wellbore;

Research sponsored by the U.S. Department of Energys Federal Energy Technology Center, under contractDE-AC21-92MC28182 with Smith International, Inc., P.O. Box 60068, Houston, TX 77205-0068;Telefax: 281-233-5205

To investigate and accommodate telemetry tools which are not dependent on the mudcolumn as the data transmitting medium. The above three sections make up a completesystem referred to as the Steerable Percussion Air Drilling System.

Since October 1992, the development of SPADS was jointly funded by Smith International, Inc.and the U.S. Department of Energy. The project was divided into two phases. Phase I of thisproject titled Steerable Percussion Air Drilling System project involved the development of asteerable hammer for the 7 7 8 , 8 12 or 8 34 hole sizes . The ongoing Phase II of this project iscalled Slim Hole Directional Drilling System, and seeks to expand the envelope of SPADS to slimhole, 4 12 , and coiled tubing applications.

The specific objectives of each phase were:

PHASE I: To develop, field test, and tool harden an approximately 6 34 diameter steerablepercussion air drilling system capable of controlling the borehole in the desired direction. Thissystem will consist of a self-rotating hammer, a diamond enhanced bit, bend sub, andaccommodations for surveying tools such as single shot, wireline steering tool or EM MWD.

PHASE II: Using the information and knowledge acquired in Phase I, develop, field test, and

tool harden a slim hole hammer approximately 3 34 diameter capable of operating on bothdrill pipe and coiled tubing. This system will similarly consist of a self rotating directionalhammer, a diamond enhanced bit, a bend sub, an orienting sub, and directional surveying orwireline steering tools.

Design Approach

SPADS is similar to conventional downhole hammers in many ways and is yet unique because ofits rotational bit-drive mechanism. In percussion drilling, two simultaneous actions arenecessary viz., the linear reciprocating motion of the piston to generate percussive energy,and the rotational motion of the bit to position the cutters onto new areas after each impact.In conventional hammer operation, the latter is achieved through the rotation of the entiredrill string. Field experience has shown that a rotational speed of 20 rpm for an impactingcycle rate of 1600 beats per minute is adequate. This translates to an angular displacement of4 12 per impact [1][2].

SPADS incorporates a unique rotational mechanism to achieve this desired rotation of 4-5between impacts. The kinetic energy of the reciprocating piston is tapped into to provide theenergy required to rotate the bit. The mechanism consists of a set of helical grooves on theouter surface of the piston that are keyed to the inner race of a clutch assembly. The clutchpermits the inner race to rotate during one traversing direction (upstroke) but locks the racefrom turning during downstroke and forces the piston to turn. As the piston is keyed to thebit it induces its rotation onto the bit. The resulting motion of the bit is an intermittentindexing type rotation.

Project Specifications

PHASE IThe design and fabrication included the following:

1. Rotational Hammers:Hammer OD: 6 34 to 6 7 8 "Hammer Length: 5 to 7 feetRotational Speed: 20 to 30 RPMOutput Torque: 1,000 ft-lb. maximum.

2. Percussion Bits: Diamond Enhanced bits for 7 7 8 , 8 12 , or 8 34 " hole sizes.3. Bend Subs: Fixed or adjustable angle bend subs, from 0.5- 3.4. Stabilizers: Near-bit and other stabilizers as required in the bottom hole assemblies.

The field testing plan consisted of:1. Straight Hole testing: To harden the capability of SPADS to drill through hard rockindependently from the drill string rotation.2. Directional Hole testing: To conduct three shallow directional field tests to evaluate buildrates as a function of bend angles and placement of stabilizers, effects of side loads andreactive torque, and system reliability and mean time between failures.3. System Field Testing: To conduct up to three high angle or horizontal field tests to provethe system capability for commercialization.

In addition, the contract also calls for the evaluation of Geoscience Electronics Corporation's(GEC) electromagnetic MWD tool as a means to telemeter directional data.

PHASE IIThe design and fabrication will include the following:

1. Rotational Hammers:Hammer OD: 3 34 Hammer Length: 5 to 7 feetRotational Speed: 20 to 30 RPMOutput Torque: 500 ft-lb. maximum.

2. Percussion Bits: Diamond Enhanced bits, as required.3. Bend Subs: Fixed or adjustable angle bend subs, as required.4. Stabilizers: Near-bit and other stabilizers as required in the bottom hole assemblies.

The field testing plan shall consist of:1. Straight Hole testing: Five to seven tests to harden the capability of SPADS to drill throughhard rock independently from the drill string rotation.2. Directional Hole testing: Three to five shallow directional field tests to evaluate build ratesas a function of bend angles and placement of stabilizers, effect of side loads and reactivetorque, and system reliability and mean time between failures.3. System Field Testing: Three to five high angle or horizontal field tests to prove thecapability of the system for commercialization.

Results

1. LAB TESTINGAn in-house testing facility for SPADS was designed and built to monitor the effect of differentdesign parameters on hammer performance. The test facility is equipped with a flow loop, a dataacquisition system, and associated instrumentation to measure different parameters that willindicate the hammer performance. During the testing, the following parameters were monitored:impact frequency, impact velocity, pressure cycle in upper and lower chambers, and bit rotation.

As a result of this testing, the performance of SPADS was significantly improved by Optimally sizing the upper and lower chambers, and controlling charging and discharging

cycles for air to maximize the impact energy and the operating frequency i.e., rate of energytransfer to the bit.

Optimally distributing the kinetic energy of the piston between linear and rotational motion, toensure adequate bit rotation under extreme conditions of torque.

2. FIELD TESTINGA number of field tests were conducted in Phase I of this project mainly in 7 7 8 and 8 12 holes.The first set of tests were conducted in straight hole to demonstrate the ability of the system todrill straight through hard rock independent of the drill string rotation at good penetration rates.During 1994-1995 a total of seven straight hole field tests were conducted in West Virginia andWest Texas during which the hammer was hardened and has demonstrated its ability to drillapproximately 1,000 ft without failure.

While the drilling machine proved its ability to drill at a penetration rate (ROP) exceeding 90 feetper hour without drill string rotation, it also showed its sensitivity to excessive weight on bit(WOB). Occasional stalling of the piston was seen during these tests. Additional lab testing wasconducted to gain better understanding about the effect of different operational variables on themagnitude of the torque required to rotate the bit. A torque sub was fabricated to monitor thistorque real time during drilling.

Among the different variables such as WOB, side loads, and different formations, the lab testingconclusively indicated that there is an optimum range of WOB corresponding to every operatingpressure at which torque is reasonably low. An increase in WOB above this optimum isaccompanied by an increase in torque and a corresponding decrease in ROP. Clearly, as torqueincreases, more energy must be imparted towards rotational motion thereby reducing impactvelocity and ROP. This torque must be kept reasonably low to ensure continuous cycling of thepiston.

A significant improvement in performance was observed when drilling with a pull down rig, whichallows WOB to be accurately controlled. A surface WOB control means such as a pull down rigis most desirable but may not always be available in the field and would probably proveinadequate in presence of drag during directional and horizontal drilling. A downhole sub, calledas Constant-WOB sub, was designed to maintain a constant hold down force necessary foruninterrupted hammer operation, while keeping the torque to a minimum. Since the hold down

force applied by this sub is proportional to the operating pressure, optimum WOB is automaticallymaintained over a wide range of line pressures.

The directional field testing phase for this project commenced in 1996. The objectives of thesetests were to demonstrate the ability of the hammer to build angle and steer the direction of thewell bore and to study the build rates and dogleg severities obtained with different BHAsconsisting of different bend angles. During the course of four tests conducted to date, thehammer has successfully completed its primary objective by demonstrating the ability to build holeangle as well as change hole direction. The magnitude of build rates and dogleg severities as afunction of the bend angle is shown in Figure 1 and compared to those obtained with aconventional PDM motor [5].

Dogleg Severities vs. Bend Angle

0

2

4

6

8

10

12

14

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Bend Angle (degrees)

Dog

leg

Seve

rity

(de

g/10

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Theoretical

Actual (Hammer)

Typical PDM

Figure 1: Build Rates with SPADS

During its build and turn sections, the hammer drilled at good penetration rates (60-120 ft/hr)comparable to a straight hole ROP with a conventional air hammer. The formations ranged fromsoft shale to hard limestone and granite (with compressive strength as high as 22,000 psi). Buildrates with SPADS were generally greater than conventional downhole motors for similar bottomhole assemblies. The reactive torque during directional field testing was low; in the range of 500-1,000 ft-lb. More importantly, this torque is cyclic in nature, not constant as observed indownhole motors. As a result, the tool face stays mostly constant, allowing easy control of thehole azimuth.

The typical BHA consisted of a diamond enhanced hammer bit, steerable hammer, bend subs(bend angles ranging from 0.5-1.5), constant WOB sub, mule shoe sub, and nonmagnetic collars.Single shot equipment was used during most of the tests to survey and orient the tool face.

The directional field tests conducted to date have shown the feasibility of drilling Pad typedirectional drilling applications, so as to eliminate expensive location costs [6]. SPADS is

currently being commercialized for directional sidetracks, build sections, and short correction runsfor directional wells in hole sizes ranging from 7 7 8 -8 34 .

Future Activities

PHASE IThe additional side load encountered in directional drilling has reduced the life of differentcomponents to a few hundred feet. Engineering efforts are focused on further hardening the toolto at least a few thousand feet for directional and horizontal drilling applications. It is our goal tocomplete Phase I by conducting the system field tests in highly deviated or horizontal holes in1997.

PHASE IIThe preliminary design of the 3 34 O.D. hammer has been completed. Fabrication of threeprototypes is currently underway. The schedule of Phase II calls for straight hole testing of thishammer with coiled tubing in the second quarter of this year. A downhole sub which allows thetool face to be oriented in coiled tubing applications will be designed and fabricated for thedirectional and horizontal field tests to be conducted in 1998. The slim hole directional drillingsystem will open doors to new directional wells as well as deepening existing wells.

References

1. Bui, H. Steerable Percussion Air Drilling System, Proceedings of the Natural Gas R&DContractors Review Meeting, U.S. Department of Energy, DOE/METC-94/1002 (DE94004065), November 16-18, 1993.

2. Bui, H., and Oliver, M., Percussion Drilling Assembly, U.S. Patent No. 5,332,136, issuedJune 21, 1994.

3. Whitley, M., and England, W., Air Drilling Operations Improved by Percussion Bit-HammerTool Tandem, SPE/IADC 13429, presented at the SPE/IADC Drilling Conference, NewOrleans, LA, March 6-8, 1985.

4. Pratt, C., Modifications to and Experience with Percussion Air Drilling, SPE/IADC 16166,presented at the SPE/IADC Drilling Conference, New Orleans, LA, March 15-18, 1987.

5. Halliburton Energy Services, Dyna-Drill Handbook, 8th Edition.6. Bui, H., Drake, D., Hairford, M., Meyers, J., and Yost, A., Directional Air Hammer can

Eliminate Expensive Location Costs, CADE/CAODC Spring Drilling Conference, April 1997(to appear).

Acknowledgments

This project was partially supported through a cost-shared contract with the U.S. Department ofEnergy since September 1992 through Contract DE-AC21-92MC28182. Their participation isgratefully acknowledged. Grateful thanks are also due to Albert B. Yost II, Federal EnergyTechnology Center, Morgantown, WV for his enthusiastic support and constructive critique atvarious times during the project. Finally, the authors wish to thank the management at SmithInternational, Inc. for providing an opportunity to bring this technology to the industry.